Potentially devastating wheat rust spreads

Since the 1950s, resistance genes bred into wheat varieties have held truly devastating stem rust epidemics in check. However, a new race of the rust, Ug99, has overcome many of those resistance genes and is marching east through southern Asia.

Ug99 first appeared in Uganda wheat in 1999 and spread to Kenya and Ethiopia during the next few years.

“At that point, many international scientists said, ‘This is something we need to check because this new race can overcome many of the effective resistances,’” said David Marshall, research leader with the USDA-ARS in North Carolina last spring.

“And that included the resistances that are in the international germ plasm out of the International Maize and Wheat Improvement Center near Mexico City. That’s alarming and this rust has become a front-burner issue.”

The spread of Ug99 through east Africa “raised a red flag and the USDA, in cooperation with CIMMYT and other international breeding centers, set up a program to identify germ plasm worldwide, based on how it fares — resistant, intermediate, or susceptible — with the new rust race,” said Marshall.

That research, done in large part with the Global Rust Initiative (see http://www.globalrust.org/), continues to move forward.

Unfortunately, so does Ug99. By last year, the rust had leapt from Africa into Yemen. Now confirmed in Iran, wind patterns suggest Ug99 could have also reached the northern Middle East, Afghanistan, India and Pakistan. Some 20 percent of the world’s wheat is grown in India and Pakistan.

“One of the interesting factors that Iran has introduced to the situation is barbary bushes, an alternate host for the stem rust fungus,” says Gene Milus, University of Arkansas professor and wheat pathologist. “That means Ug99 will likely hybridize with the stem rust isolates that are native to Iran.”

The result: isolates coming out of Iran will probably be different than the Ug99 that went into the country.

“That’s due to the sexual recombination and will mix up the genotype. If indeed that happens, the thinking is there may be even more resistance genes overcome by new races coming out of Iran.”

Destabilizing factor?

The wheat varieties being grown in Afghanistan are susceptible to Ug99. Even though there isn’t huge wheat acreage there, many Afghanis — as with African producers — are subsistence farmers.

“This disease could really mess things up in that region of the world,” says Milus. “Adding another volatile element in that area could destabilize it even further. No one wants that.”

Eventually, the new stem rust will end up in the United States.

“Will it arrive on the trade winds — we get winds out of China, across the Pacific Ocean and into the United States. Or will it arrive via clothing? Obviously, there is a lot of traffic between the United States and the Middle East these days. There is a serious possibility this rust could arrive aboard an airplane,” says Jim Peterson, wheat breeder at Oregon State University and chair of the National Wheat Improvement Committee.

“We don’t know how much time there is before it’s found here. Months? A decade? But it is multiplying and moving well with the aid of susceptible wheat varieties.”

The immediate worry is with wheat in the subcontinent. At least 75 percent of the region’s major production area is susceptible to Ug99.

“It is critical to get new disease resistances integrated into their varieties and deployed as quickly as possible,” says Peterson. “If that doesn’t happen, there could be serious food shortages in those areas.”

“As you can imagine, Ug99 has been a top concern of ours and the National Association of Wheat Growers for several years. We’ve continued to communicate those concerns to Congress and ARS to try and promote research funding to address this critical vulnerability in the United States and around the world.”

Unfortunately, U.S. funding is largely focused elsewhere.

“The USDA-ARS is certainly taking a leadership role in addressing this exposure. They’ve sponsored and funded screening of U.S. material — germplasm, varieties, experimental lines — in Kenya over the last three years.”

That work has produced a “tremendous data set” to understand both vulnerability and what resistances can be deployed against the rust.

“That’s the good news. The bad news is we’ve been going to Congress for the last three years trying to raise awareness of Ug99 and promote additional funding for both ARS and state research programs that breed wheat varieties. State pathologists are included in those efforts, obviously.

“Our efforts to increase funding have so far been unsuccessful. We’re optimistic growing awareness — along with recent, acute concerns over world food security — might spur additional research funding.”

Does the new farm bill provide any funding? “The ink on that is still wet,” says Peterson. “But to my knowledge, it doesn’t. The word is there’s been no infusion of funds into this rust research.”

Milus is disappointed that Congress doesn’t understand what’s at stake with the disease. “On top of everything, our government has cut USAID (United States Agency for International Development) funding for just this sort of work. For this fiscal year, 2008, the money that’s been providing base support for international research centers — not just for wheat, but all crops — has been cut to zero. Congress cut next year’s funding, as well.”

This is shortsighted in the extreme, insists Milus. “The disease can be erratic but under the right environmental conditions, there can be 100 percent loss in infected fields. It can be devastating.”

Both men laud the Bill and Melinda Gates Foundation, which pledged $26.7 million towards monitoring and combating the rust for the next three years.

“That funding is a critical message to others about how important this is and how exposed many developing nations are to Ug99,” says Peterson. “There are solutions, here. From our standpoint, scientists need resources available so we can respond quickly. That’s been a big challenge, thus far. I hope timing is in our favor,” says Peterson.

Rust combat

A bit of historical perspective: until the 1960s, stem rust was a major disease in the Midwest. Some years, 30 to 50 percent of North Dakota’s wheat crop was lost to the disease.

Currently, “a major portion of the world’s wheat has a high vulnerability to the disease,” says Peterson. “Fungicides can be of help, but they aren’t a silver bullet — particularly in developing nations that have difficulty accessing fungicides, much less applying them.”

And how in a developing nation could vast wheat acreages be treated? Not many countries have aerial applicator fleets.

“The fear is we’ll have a year when the environment is perfect for the disease and it takes off. I don’t know how small farmers would be able to stay ahead of it — even if the fungicides are available to them.”

In such circumstance, jugs of fungicides could be sitting outside farmers’ doors, “but do they have the means, infrastructure and education to apply fungicides at the proper time, proper stage of development? Perhaps they could minimize the damage, but it is certainly unlikely they could eliminate the disease pressure.”

Kenyan farmers are spraying fungicides two or three times during the wheat growth cycle. “They’re under intense rust pressures — stem rust first, but also leaf and stripe. So, they have to use high levels of fungicide just to get a crop. That should be a warning to everyone.”

As it was in the 1950s, the answer to the new stem rust is variety resistance.

“UG99 took out many of the core resistance genes we’ve relied on for decades. That has left us extremely vulnerable. There are a small number of genes still effective. Right now, the strategy is to, as quickly as possible, move those into adapted varieties and get them into farmers’ hands.”

The International Maize and Wheat Improvement Center (CIMMYT) is “working frantically” to do just that, says Milus. “They have a small number of experimental lines already on seed increase with genes they think will provide effective resistance. They’ll deploy those in India, Pakistan and parts of Africa. But it takes time. Just to increase seed stocks to cover a couple of countries takes two or three years.”

Predicting the rust’s movement is very tricky, says Peterson. There have been attempts to match up information on wind flow and the disease’s progress out of Africa.

“CIMMYT has done a tremendous job predicting movement based on wind flow. But one event can throw everything off. Last year, one major wind event essentially moved the rust from Yemen into Iran. “

Disturbingly, additional wind events have since been recorded that “could have already moved the disease into India and Pakistan. That isn’t a definite — and nothing has been found on the ground. But the model says it’s possibly there.”